1. Field of the Invention
The present invention relates generally to liquid crystal devices arranged in an image bar configuration for use in electrophotographic printers and, more particularly, to an improved optical shield for such liquid crystal image bars, together with a method of fabrication thereof.
2. Description of the Prior Art
In a typical transmissive type liquid crystal device, a thin layer of liquid crystal material sandwiched between parallel transparent glass substrates bearing transparent patterned electrodes on their inner confronting surfaces. At least one polarizer is located on the outer surface of one of the glass substrates and a light source is spaced from the device and directs light therethrough. By selectively applying an electric field across the layer of liquid crystal material by means of an A.C. voltage to the electrodes, the transmissivity of liquid crystal device be changed for passing or blocking light in accordance with the electrodes addressed by the voltage.
U.S. Pat. No. 4,595,259 Perregaux discloses a transmissive type liquid crystal device arranged in an image bar configuration. The image bar has a nematic liquid crystal material sandwiched between elongated glass substrates with an array of transparent electrodes on the confronting surfaces thereof, crossed polarizers, and means for slectively energizing the electrodes with a single frequency voltage. This image bar configuration functions as a linear array of dot shutters, and produces latent electrostatic images on the printer's photoconductive member one line at a time in response to receipt of digitized data signals. During the operational mode, all of the image bar shutters are energized, and in this electrically driven state, the image bar shutters are in their state of lowest transmission. The latent image is produced by selective erasure of precharged background areas on the member. For the erasure to occur, interruption of the driving voltage to selected shutters in response to the data signals cause the selected shutter to enter a light transmitting transient state. The response time in the transmissiveness is arranged to about one millisecond. Consecutively erased spots by the same shutter requires the momentary reapplication of the driving voltage. Because the function of the image bar is relatively independent of temperature and gap thickness, it is very cost effective. A shield is used in this patent to substantially eliminate stray light from entering the liquid crystal material, and hence the photoreceptor. The light is formed from a typical opaque material and generally has a thickness of 2 to 10 micrometers. The formation of the shield material and its subsequent delineation of a slit therein is a separate process completely different from the other fabricating processes for the liquid crystal image bar.
A copending application entitled "Transient State Liquid Crystal Image Bar With Contrast Enhancement", U.S. Ser. No. 937,806, filed Dec. 4, 1986, by Dir et al, and having the same assignee as this invention, discloses an improvement over the above patent to Perregaux. Enhanced contrast with lower required driving voltage is achieved by an optical compensator or a thin prism placed between the polarizer and the liquid crystal cell. In another embodiment, a quarter-wave plate is placed between the cell and the analyzer. A slight rotation of the analyzer about its optical axis increases extinction of light exiting from the cell.
U.S. Pat. No. 4,311,773 to kaneko et al discloses a method of producing color filters. The method includes coating a photosensitive material on a substrate and exposing the coating to ultra-violet light through a predetermined pattern. A solvent is used to remove select portions of the photosensitive coating. The remaining portions of the photosensitive coating are then dyed into a desired color.
U.S. Pat. No. 4,370,405 to O'Toole et al discloses a multi-layer photoresist process utilizing an absorbent dye. A resist layer is deposited on a substrate which contains a dye layer on its surface. The dye layer absorbs light of the wavelength used to expose the top layer of the resist material, therefore, protecting the substrate from unwanted exposure. The aim of this patent is to improve line resolution and uniformity. The photoresist line width control problems due to scattering and reflection from the substrate-resist interface can be solved by reducing or eliminating the substrate reflection. Line width control problems arising from surface topography are eliminated by using a multi-level resist process with the lower level of the photoresist containing an absorbent dye, this dye significantly reduces the substrate-resist interface reflections problem.
U.S. Pat. No. 4,487,481 to Suzawa discloses a liquid crystal display panel having a backlight for improving high brightness, uniformity of illumination intensity, and small thickness among other things. The liquid crystal display panel has a side window for allowing light to pass through the liquid crystal layer. Several embodiments of a backlighting technique are disclosed.
U.S. Pat. No. 4,567,132 to Fredericks et al discloses a photolithographic process for forming precise aperture size windows in a photoresist layer. The process utilizes two photoresist layers. The layers are exposed to ultra-violet light and then baked. The layers are developed by using a solvent in which the first layer is soluble and the second layer is insoluble. The characteristics of the differing photoresist layers allows for precise control of the cross-sectional dimension of the windows in the photoresist layer. U.S. Pat. No. 4,568,631 to Badami et al discloses a photolithographic process for producing resist lines in the micron and submicron range. A photoresist is applied to a substrate, exposed to ultraviolet light, and then developed using a solvent. The photoresist is then baked, exposed a second time, and developed using a solvent. This process results in thin resist lines which can be used to form narrow isolation trenches.
U.S. Pat. No. 4,589,732 to Shiraishi et al discloses a liquid crystal image bar containing an array of light valves with a metal mask for restricting the passage of light through the transparent electrodes. U.K. Pat. No. 2,125,202 to Masaki et al discloses a liquid crystal cell comprising a liquid crystal between a pair of electrode supporting plates wherein at least one of the electrode supporting plates is provided with a light intercepting mask which is formed from a water-soluble resin and colored on the area other than the pattern electrode area. The liquid crystal display element may be used in a data recording device for photographically recording some data indicated by the selected segments of the display electrodes onto, for example, a film. The mask is nonconducting so that it may be placed in direct contact with the patterned electrodes. The intercepting mask is formed such that it is transparent where it contacts the transparent electrodes and the remainder of the mask is dyed so that it is non-transparent.
None of the optical shields for the prior art liquid crystal panels and image bars provide an optical shield having a thickness on the order of two microns that is compatible with the liquid crystal cell fabrication, very inexpensive to process, and provides sharp imaging apertures in the optical stop.